Arterial thrombosis resulting from disruption of atherosclerotic plaques is a leading cause of death due to acute thrombosis. Previous studies have demonstrated that hyperlipidemia significantly increases the risk of arterial thrombosis via activation of coagulation. Tissue factor (TF), the cellular activator of the clotting cascade, is induced when monocytes are exposed to oxidized lipoproteins. Activation of monocytes results in the subsequent release of TF-positive microparticles (MPs), which represent a source of circulating TF in hypercoagulable states. A class of minimally oxidized lipoproteins, including the phospholipid oxidation product oxPAPC, have previously been demonstrated to activate the Toll-like 4 (TLR4) receptor. We will test the hypothesis that oxidized lipoproteins are associated with activation of the coagulation system by binding to TLR4 on monocytes and inducing the expression of TF and the release of TF-positive MPs. My proposal is divided into two aims. Aim 1 will determine the mechanism by which oxPAPC induces TF expression in monocytes and the generation of TF-positive MPs. We will measure TF expression in oxPAPC treated monocytes/macrophages. We will utilize mouse monocytes/macrophages deficient in TLR4, and human monocytes with an inhibitory anti-TLR4 antibody. Aim 2 will delineate the role of monocyte-derived MP TF in the activation of coagulation in hyperlipidemic mice. The effects of TF blockade on thrombin-antithrombin (TAT), a marker of activation of coagulation, will be determined using TF neutralizing antibody in a mouse model of hyperlipidemia. Further analyses will be performed using bone marrow transplantation of either low TF or myeloid deficient Lys-M TF, into LDLr-/- mice. This will determine if reducing monocyte-derived, TF-positive MPs reduces TAT and time to occlusion in a mouse model of arterial thrombosis. These studies will increase our understanding of how lipidemia modulates levels of circulating TF and whether the major source of TF-positive MPs is monocytes. Over 80,000,000 people in the Unites States suffer from one or more forms of cardiovascular disease (CVD) and over 850,000 of these cases result in death every year, making it the number one killer in America. Plaque disruption and subsequent arterial thrombosis is a critical event in atherosclerosis resulting in acute vascular syndromes, such as myocardial infarction and stroke. Increased understanding of the mechanisms of thrombosis in CVD will enable the scientific and medical communities to make advances in therapeutics and treatment strategies.